(I combine this in my preflight)
And post-flight....
Been a long time since I flew the 206B, but from memory I was told (by a Bell prod. test pilot) that by turning the blades backwards after landing to tie the blades down would help against the coking, and during pre-flight it would tell you if you would get the blades to turn during enginestart.... If coked, you would only get a bloody high TOT with no blade-turning
We always did 2 min at idle before shutdown.
The 350, I've been instructed to 30 sec, but some companies want 1 min as well.